Thick film materials have been used for packaging applications in the microelectronics industry. Their acceptance in the industry, however, has not been widespread because thin film technology using silicon photolithographic processes has been better adapted for high performance requirements, such as microwave and other radio frequency applications. The thick film substrates also have poor geometrical resolution and a very high dielectric constant. Via formation has not been adequate, and typically greater than 200 microns.
In recent microelectronic packaging designs, there has been a greater need for high performance materials and processes that allow finer geometries and circuit layout without sacrificing performance. As a result, for critical applications as in the fabrication of microstrip transmission lines and similar structures, thin film silicon and other related thin film technology has been used instead of thick film processing technology.
Three major approaches for fabricating microstrip transmission lines and similar structures have been thin film, thick film and polymer technology, i.e., soft board. These technologies have some drawbacks with regard to current and future applications. Thin film technology has become established for use with microwave circuit and component technology, but it is expensive because of its typical use of sputter coating, which is also wasteful in material. Additionally, an adhesion layer is required between a sputtered line and a substrate to create an insulator at the interface with the substrate, where current flows in the microstrip line. Because a thin film material follows the surface finish of a substrate, losses increase, which are significant with increasing frequency. Thus, an expensive 99.6% polished alumina has been used for thin film applications, and increases even more the cost of a thin film microwave or radio frequency circuit. Even then, this type of circuit will suffer losses due to the presence of the adhesion layer.
Polymer technology is used increasingly for microwave circuits because it has a lower cost. These lower cost polymer materials, however, are high in dielectric loss and have limited component geometries and some environmental limitations. Some of the more advanced materials overcome these drawbacks, but are even more expensive. Even though geometrical constraints can be overcome by the use of a build-up technology, the costs increase.
Thick film technology, such as the well known green tape technology and other similar materials technology, is little used at the present for microwave circuit applications and similar radio frequency applications, even though it is an inexpensive medium. Thick film circuit technology can resolve many microstrip lines, but it occurs with poor edge definition and a relatively rough conductive surface. For some components, such as an edge coupled filter, it does not produce the fine gaps required for the design. Also, the dielectric materials generally are high in dielectric constant (seven or greater) and have a dielectric loss of 0.01, i.e., about 1%. The low cost of the thick film materials, however, would make them advantageous for use in microelectromechanical structures used with radio frequency applications, if a means could be found to overcome the drawbacks as indicated above.